The present invention relates to a rotary head assembly and more particularly a lubrication system for bearing means of a rotary head of a video tape recorder to be referred to as "VTR" in this specification for brevity.
The rotary head assembly for VTR must satisfy the following requirements among other things:
(1) The desired rotational speed of the upper head carrying the video signal recording and reproducing heads must be maintained with a high degree of accuracy.
(2) Lateral vibration and/or rocking of the axis of rotation of the upper head must be minimized.
The recent tendency of VTR is towards recording at a higher density and more compactness, so that the recording time may be increased and the operation of the portable VTR may be facilitated.
The above requirements cannot be met by the conventional rotary head assembly wherein the rotary shaft carrying the upper cylinder which in turn carries video signal recording and reproducing heads is supported by rolling-contact bearings which are spaced apart from each other. For instance, the accuracy of rotation is adversely affected by the poor running accuracy of the rolling contact bearings, the vibrations caused by the elasticity of their inner and outer races, plays between the component parts and so on.
Instead of rolling contact bearings, oilless bearings have been also used, but they are the so-called boundary lubrication bearings so that excessive play between the bearings and journals results in variations or inaccuracy in rotational speed.
In order to overcome the above and other problems, hydraulic bearings have also been used, but they have some problems to be described below.
(1) It is difficult to design and construct a rotary head assembly which is compact in size and may be fabricated at low cost.
Prerequisites for home electronic equipment such as VTR are compactness and mass production at low cost. The rotary head assemblies for industrial VTR's incorporating the static pneumatic bearings have been long produced, but they are not adapted to be incorporated into home VTR because a compressor, which is a compressed air source, and a complex pneumatic control circuit must be included. The dynamic pneumatic bearings need no compressor, but their effects (high pressure and rigidity) are attained only at extremely high rotational speeds of the order of 1800-3600 rpm so that they cannot be used in home VTR's.
(2) When an oil is used as a lubricant, it becomes extremely difficult to seal or contain it for a long time.
While the lubricant for machine tools may be replaced or supplemented at any time, the oil for the hydraulic bearing of the VTR rotary head assembly must be completely sealed so that no supplement will be needed. When the oil leaks, the lubrication effects (rigidity and load capacity) of the hydraulic bearing are degraded so that the rotational speed varies. Furthermore the leakage of the lubricating oil will adversely affect the operation of VTR. When the leaked lubricating oil attaches to the heads and consequently to the tape, the dust is easily accumulated so that the correct recording and reproduction will become impossible due to dropout error. That is, the FM carrier is partially lost, resulting in dropouts.
There have been used (a) contact type seals and (b) non-contact type seals so as to prevent the leakage of lubricating oil. For instance, oil seals, O-rings, mechanical seals and so on belong to the contact type seals. They are directly fitted over the rotary shafts and exert the pressure of contact to them so that they place a considerable load on them. As a result, the highly precise rotational functions (the uniform torque and uniform thickness of oil film) of the hydraulic bearing cannot be attained. The rotary head assembly of VTR is driven by a small, low-torque direct drive motor (to be referred to as "DD motor" in this specification), and the variations in rotational speed must be less than 0.002%. Therefore a load placed on the rotary shaft must be as lowest as possible. Thus the application of the contact type seals to the rotary head assembly of VTR is difficult.
The non-contact type seals utilize the centrifugal force exerted to a liquid or the pumping action of a viscous liquid as in the case of the visco seals. During the rotation, no leakage occurs, but when the rotary shaft stops the oil leakage problem arises.
In addition to the contact and noncontact type seals, the magnetic seal utilizing a magnetic liquid has been also used to prevent the leakage of a lubricant. For instance, U.S. Pat. No. 4,043,612 discloses a hydraulic bearing for industrial VTR's which employs a magnetic liquid as a lubricant, but this hydraulic bearing is not adapted for use in the rotary head assembly for a home VTR.